One of the most striking changes in the cardiovascular system which accompanies advancing age is the prolongation of the duration of systole. This age-associated prolongation occurs in man, lower mammals, and in rodents, and apparently represents an age related change in cardiac muscle across species. However, advancing age in rodents is also often accompanied by progressive decline in renal function, making it difficult to partition cardiovascular changes into truly age-related versus renal related declines. In order to determine whether myocardial function in senescent (24 mo) male Wistar rats varies with SUN, we measured function of isolated cardiac muscle, from animals with higher and lower SUN. Muscles were bathed in normal Ringer's bicarbonate solution ((Ca) = 1.0 mM) and stimulated to contract 24 min-1 at 30% at Lmax. The mean plus/minus SEM, SUN, resting tension (RT), developed twitch tension (TT), maximum rate of tension rise (dt/dtmax) and contraction duration (CD) in the two groups. Thus, these contractile properties of isolated cardiac muscle do not depend on SUN. Additionally, the maximum contractile response to isoproterenol (10-5M) did not vary with SUN (dt.dt = 34 plus/minus 6 vs 21 plus/minus 9% control in the high and low SUN group respectively. We conclude that contractile function is cardiac muscle from senescent rats is not affected by the levels of SUN encountered in these rats.